1. Field of the Invention
The invention relates to the field of semiconductor metrology, and in particular, to an improved system and method for performing various electrical measurements using combined corona and microwave techniques.
2. Related Art
Various electrical measurements can be done on a semiconductor wafer to determine the performance characteristics of the devices formed on that semiconductor wafer. Exemplary electrical measurements include capacitance, sheet resistance (i.e. the resistance of very thin doped regions in a substrate), and mobility (i.e. the relationship between resistivity and charge density (resistivity/charge density) associated with the surface of stressed (using compression or tensile forces) silicon). Each of these electrical measurements has conventionally been performed using different techniques (and correspondingly different equipment). Unfortunately, each of these techniques has entailed significant disadvantages.
FIG. 1A illustrates a technique 100 that can measure the capacitance versus voltage of a gate film (e.g. an oxide or dielectric). In technique 100, a series of corona charges can be deposited on the gate film in step 101. FIG. 1B illustrates an exemplary corona bias in which a corona charge generator 112A can produce ions 112 that deposit precise amounts of charge 113 on a gate film 111. In one embodiment, the area of gate film 111 designated for charge deposition can be defined by corona blocking surfaces 114. Gate film 111 can be formed on a substrate 110 (e.g. a p silicon).
The deposited charges 113 on gate film 111 form a virtual electrode of an MOS capacitor, wherein the gate film 111 forms the dielectric and substrate 110 can form the other electrode of the MOS capacitor. Note that the deposited charges 113 contact the surface of gate film 111 with very low kinetic energy, thereby ensuring no damage to gate film 111.
After the deposition of corona charges, a step 102 (FIG. 1A) can measure the surface voltage of gate film 110 with a Kelvin probe. FIG. 1C illustrates a simplified Kelvin probe 120 that can measure a surface voltage associated with deposited charges 113. Specifically, Kelvin probe 120 essentially functions as a non-intrusive voltmeter with virtually infinite input impedance.
By incrementally depositing more charge and repeating the voltage measurements of steps 101 and 102, step 103 (FIG. 1A) can generate a charge-voltage (Q-V) curve. FIG. 1D illustrates an exemplary Q-V curve 130. In turn, a capacitance-voltage (C-V) curve can be derived from Q-V curve 130 in step 104 (FIG. 1A). Specifically, referring to FIG. 1D, the capacitances can be determined from a slope of an accumulation region 131 in Q-V curve 130. FIG. 1E illustrates an exemplary C-V curve 140 derived from Q-V curve 130. Unfortunately, because of the small area associated with charge deposition, it is difficult to get an accurate measure of the amount of charge deposited during step 101, thereby preventing an accurate measurement of the capacitance in step 104.
FIG. 2 illustrates a technique 200 for measuring the mobility of a substrate under a gate. In this technique 200, step 201 constructs a complete transistor connected to a pad. Step 202 then probes the pad with an electrical testing system. Technique 200, albeit accurate, has the disadvantage requiring actual construction of the complete transistor and its connected pad before the measurement can be made.
FIG. 3A illustrates a technique 300 for measuring the sheet resistance of doped regions in the semiconductor (i.e. implants). In technique 300, step 301 uses either a microwave probe or, alternatively, a four-point probe to measure the sheet resistance. FIG. 3B illustrates an exemplary microwave probe 310 including a micro-coaxial cable 311 having a center conductor 315 that provides a microwave signal and two peripheral elements 313 (e.g. providing signal-ground (S-G)). In this embodiment, microwave probe 310 can include a sleeve 312 for adjusting the pitch between center conductor 315 and peripheral elements 313.
Unfortunately, microwave probe 310 is sensitive to both the sheet resistance of the shallow implanted region as well as the sheet resistance of the substrate below the implanted layer, thereby making separation of the two effects difficult. One disadvantage of using a 4-point probe for measuring sheet resistance is that the 4-point probe must physically contact the wafer, thereby producing undesirable particles. Additionally, the 4-point probe requires an un-patterned region of several millimeters in dimension, which is not commercially viable for production wafers. Yet further, current implants of ultra-shallow junctions can be too shallow to allow an accurate and repeatable measurement, i.e. the tips of the 4-point probe can easily puncture through the entire of the implanted region into the substrate.
Therefore, a need arises for metrology techniques to accurately and efficiently perform electrical measurements on semiconductors.